PLC Components and Systems

perperde by : habtamu zewdie

2.1. Building blocks of PLC system

• The PLC is basically a programmed interface between the

field input elements like limit switches, sensors, transducers,
pushbutton etc and the final control elements like actuators,
solenoid valves, dampers, drives, LEDs, hooters etc.
• This interface called as programmable logic controller consists
of the following:
 Input/output modules
 Input/output circuits
 CPU with processor and program memory
 Bus systems
 Power supply
 POWER
SUPPLY

I M O M
N O
P D U O
U U PROCESSOR T D
T L P U
From E U L
To
SENSORS T E
OUTPUT
Pushbuttons, Solenoids,
contacts, contactors,
limit switches, alarms
etc. etc.
PROGRAMMING
DEVICE
2.2. Input/output Module
•The I/O interface section of a PLC connects it to
external field devices.
•The main purpose of the I/O interface is to condition the
various signals received from or sent to the external input
and output devices.
•Input modules converts signals from discrete or analog
input
devices to logic levels acceptable to PLC’s processor.
•Output modules converts signal from the processor to
levels
capable of driving the connected discrete or analog output
devices.
•The voltage and current signals generated by the sensors,
transducers, limit switches, pushbuttons etc are applied to
the terminals of the input module.
• The input module helps in the following ways:
 It converts the field signal into a standard control signal for
processing by PLC. The standard control signal delivered
by input module could be 5V or 9V whereas the field
signal received by it, could be 24V DC, 230V AC or 115V
AC.
 If required, it isolates the field signal from the CPU.
 It sends one input at a time to CPU by multiplexing action.
 Depending upon the nature of input signal coming from the
field the input module could be:
 Analog input module
 Digital input module
• The typical analog current input modules are 4 to +- 20mA, 0 to
+- 50 mA and analog voltage input modules are 0 to
+- 50mV, 0 to +- 500mV and 0 to +- 10 V
• The typical digital input modules are 24V DC, 115V AC and
230V AC.
• The output module acts as a link between the CPU and the output
devices located in the field.
• The field devices could be relays, contactors, lamps, motorized
potentiometers, actuators, solenoid valves, control valves
dampers etc. these devices actually control the process.
• The out put module converts the output signal delivered by CPU
into an appropriate voltage level suitable for the output field
device. The voltage signal provided by CPU could be 5V or 9V,
but the output module converts this voltage level into say 24V
DC, or 115V AC or 230V AC etc.
• Thus the output module on receiving signal from the processor
switches voltage to the respective output terminals. This
makes the actuators (i.e. contactors, relays etc.) or indicating
lights etc. connected to the terminal to become ON or OFF.
• Like input module, an output module could be an analog or
digital.
• The selection is based on the voltage rating of the field output
devices. If the output device is analog then analog output
module is required and if it is digital like contactor coil or a
lamp then digital output module is required.
• Typical analog output modules have the ratings of 4mA to +-
20 mA or 0 to +-10V and the digital output modules have 24V
DC, 115V AC and 230V AC output.
DC INPUT MODULE

IS NEEDED TO:
· Prevent voltage
USE TO
DROP THE transients from
VOLTAGE damaging the
TO LOGIC processor.
·Helps reduce
LEVEL
the effects of
electrical noise

Current Buffer,
FROM Limiting Filter,
OPTO- TO
INPUT Resistor ISOLATOR hysteresis
PROCESSOR
DEVICE Circuits
AC INPUT MODULE

IS NEEDED TO:
CONVERTS THE AC · Prevent voltage
INPUT TO DC AND transients from
DROPS THE damaging the
VOLTAGE TO LOGIC processor.
LEVEL ·Helps reduce
the effects of
electrical noise

Rectifier, Buffer,
FROM Resistor Filter,
OPTO- TO
INPUT Network ISOLATOR Hysteresi
PROCESSOR
DEVICE s Circuits
DC / AC OUTPUT MODULE
IS NEEDED TO:
· Prevent voltage
transients from
damaging the
processor.
·Helps reduce
the effects of
electrical noise

Amplifier
FROM RELAY
TTL(trans OPTO- TO
PROCESSOR ISOLATOR TRIAC
istor- OUTPUT
X’SISTOR
transistor DEVICE
logic)
Circuits
2.3. Input/output Circuits

Different Types of I/O Circuits

1. Pilot Duty Outputs

Outputs of this type typically are used to drive high-current
electromagnetic loads such as solenoids, relays, valves, and
motor starters.

These loads are highly inductive and exhibit a large inrush

current.

Pilot duty outputs should be capable of withstanding an

inrush current of 10 times the rated load for a short period of
time without failure.
2. General - Purpose Outputs
These are usually low- voltage and low-current and are used
to drive indicating lights and other non-inductive loads. Noise
suppression may or may not be included on this types of
modules.

3. Discrete Inputs
Circuits of this type are used to sense the status of limit
switches, push buttons, and other discrete sensors. Noise
suppression is of great importance in preventing false
indication of inputs turning on or off because of noise.
4. Analog I/O

Circuits of this type sense or drive analog signals.

Analog inputs come from devices, such as thermocouples,

strain gages, or pressure sensors, that provide a signal voltage
or current that is derived from the process variable.
Standard Analog Input signals: 4-20mA; 0-10V

Analog outputs can be used to drive devices such as

voltmeters, X-Y recorders, servomotor drives, and valves
through the use of transducers.
Standard Analog Output signals: 4-20mA; 0-5V; 0-10V
5. Special - Purpose I/O

Circuits of this type are used to interface PLCs to very specific

types of circuits such as servomotors, stepping motors PID
(proportional plus integral plus derivative) loops, high-speed
pulse counting, resolver and decoder inputs, multiplexed
displays, and keyboards.

This module allows for limited access to timer and counter

presets and other PLC variables without requiring a program
loader.
 OUTPUTS

INPUTS MOTOR

CONTACTOR
LAMP

PUSHBUTTONS
PLC
 Allen-Bradley 1746-1A16

L2 I= Input
L1
Module
I:2 slot # in rack

0
P. B SWITCH Module
Terminal #
Address I:2.0/0
LADDER PROGRAM

INPUT MODULE

WIRING DIAGRAM
 CONTACTOR
L1 N.
L2 O

MOTOR
L2
C •SOLENOI
L1 D
FIELD •VALVES
•LAMP
WIRING
•BUZZER

OUTPUT MODULE
WIRING
L1 O:4

L2

CONTACTOR 0

LADDER PROGRAM
Discrete Input

A discrete input also referred as digital input is an input that is either ON or

OFF are connected to the PLC digital input. In the ON condition it is referred to
as logic 1 or a logic high and in the OFF condition maybe referred to as logic o
or logic low.

Normally Open
Pushbutton
Normally Closed
Pushbutton
Normally Open switch

Normally Closed switch

Normally Open contact

Normally closed contact

 IN

OFF PLC
Logic 0
Input
Module
24 V dc

IN

OFF PLC
Logic 1
Input
Module
24 V dc
Analog Input

An analog input is an input signal that has a continuous

signal. Typical inputs may vary from 0 to 20mA, 4 to 20mA
or 0 to10V. Below, a level transmitter monitors the level of
liquid in the tank. Depending on the level Tx, the signal to the
PLC can either increase or decrease as the level increases
or decreases.

Level Transmitter IN

PLC
Analog
Tank Input
Module
Digital Output

A discrete output is either in an ON or OFF condition. Solenoids,

contactors coils, lamps are example of devices connected to the
Discrete or digital outputs. Below, the lamp can be turned ON or OFF by the PLC
output it is connected to.

OUT

PLC
Lamp
Digital
Output
Module
Analog Output

An analog output is an output signal that has a continuous

signal. Typical outputs may vary from 0 to 20mA, 4 to 20mA
or 0 to10V.

Electric to pneumatic transducer

OUT
E Supply air
PLC 0 to 10V P

Analog
Output
Module
Pneumatic control valve
2.4. CPU with processor and program memory
 Central Processing Unit
• The central processing unit or CPU consists of the following blocks.
 Arithmetic Logic unit
 Program memory
 Process image memory (i.e. internal memory of CPU)
 Internal timers and counters
 Flags
 The brain of CPU is its microprocessor/microcontroller chip.
• The main function of the microprocessor is to analyze data coming from
field sensors through input modules, make decisions based on the user’s
defined control program and return signal back through output modules to
the field devices. Field sensors: switches, flow, level, pressure, temp.
transmitters, etc. Field output devices: motors, valves, solenoids, lamps, or
devices.
• The working of CPU is fully controlled by the instructions/ program stored
in “user program memory”.
• The user program directs and controls the CPU’s working.
• The program is prepared by the user based on the control logic
required for the control and automation task.
 Memory
• The most important characteristic of a programmable controller
is the user’s ability to change the control program quickly and
easily. The PLC’s architecture makes this programmability
feature possible
• The memory system is the area in the PLC’s CPU where all of
the sequences of instructions, or programs, are stored and
executed by the processor to provide the desired control of field
devices.
• The memory sections that contain the control programs can be
changed, or reprogrammed, to adapt to manufacturing line
procedure changes or new system start-up requirements.
The memory system in the processor module has two parts:
a system memory and an application memory
•System memory includes an area called the EXECUTIVE,
composed of permanently-stored programs that direct all system
SYSTEM activities, such as execution of the users control program,
communication with peripheral devices, and other system activities.
•The system memory also contains the routines that implement the
PLC’s instruction set, which is composed of specific control
functions such as logic, sequencing, timing, counting, and
arithmetic.
•System memory is generally built from read-only memory devices.
APPLICATION

•Data Table •The application memory is divided into the data table area and
•User Program user program area.
•The data table stores any data associated with the user’s control
program, such as system input and output status data, and any
stored constants, variables, or preset values. The data table is
where data is monitored, manipulated, and changed for control
purposes.
•The user program area is where the programmed instructions
entered by the user are stored as an application control program.
 Memory Types
• Memory can be separated into two categories: volatile and
nonvolatile.
1. Volatile.
•A volatile memory is one that loses its stored information
when power is removed.
•Even momentary losses of power will erase any
information
stored or programmed on a volatile memory chip.
•It is easily altered and quite suitable for most
applications
when supported by battery backup and possibly a disk
copy
of the program
Common Type of Volatile Memory
RAM. Random Access Memory(Read/Write)
 •The words random access refer to the ability of any
location (address) in the memory to be accessed or used.
•RAM memory is used for both the user memory (ladder
diagrams) and storage memory in many PLC’s.
•RAM memory must have battery backup to retain or protect
the stored program.
Several Types of RAM Memory:
1.MOS
2.HMOS
3.CMOS
•The CMOS-RAM (Complimentary Metal Oxide
Semiconductor)
is probably one of the most popular. CMOS-RAM is popular
because it has a very low current drain when not being accessed
(15microamps.), and the information stored in memory can be
retained by as little as 2Vdc.
2. Non-Volatile
•Has the ability to retain stored information when power is
removed, accidentally or intentionally. These memories do not
require battery back-up.
Common Type of Non-Volatile Memory
ROM, Read Only Memory
•Read only indicates that the information stored in memory can
be read only and cannot be changed. Information in ROM is
placed there by the manufacturer for the internal use and
operation of the PLC.
PROM, Programmable Read Only Memory
•Allows initial and/or additional information to be written into
the chip.
•PROM may be written into only once after being received
from
the PLC manufacturer; programming is accomplish by pulses
•The current melts the fusible links in the device, preventing it
from being reprogrammed. This type of memory is used to
prevent unauthorized program changes
EPROM, Erasable Programmable Read Only Memory
•Ideally suited when program storage is to be semi-
permanent or additional security is needed to prevent
unauthorized program changes.
•The EPROM chip has a quartz window over a silicon
material that contains the electronic integrated circuits.
This window normally is covered by an opaque material,
but when the opaque material is removed and the circuitry
exposed to ultra violet light, the memory content can be
erased.
•The EPROM chip is also referred to as UVPROM.
 EEPROM, Electrically Erasable Programmable Read Only
Memory
• Also referred to as E2PROM, is a chip that can be programmed
using a standard programming device and can be erased by the
proper signal being applied to the erase pin.
• EEPROM is used primarily as a non-volatile backup for the
normal RAM memory. If the program in RAM is lost or
erased, a copy of the program stored on an EEPROM chip can
be down loaded into the RAM
2.5. Arithmetic Logic Unit

• ALU is the “organizer” of the PLC.

• The following operations are carried out by ALU.
 It organizes the input of external signals and data.
 It performs logic operation with the data.
 It performs calculation
 It takes account of the value of internal timers and counters
 It takes account of the signal states stored in the flags.
 It stores the signal states of the input in the “process input image”
(internal memory of CPU) before each program scanning cycle.
 It stores the result of the logic operation in the “process output image”
(internal memory of CPU) during the program scan.
 It organizes the output of the result.
2.6. Power supply
• The power supply module generates the voltages required for
the electronics modules of the PLC from the main supply.
• Special Functions Modules.
• In addition to the above listed modules, the other frequently
used modules for special functions are interface modules,
communication processor, and counter modules etc. these can
be selected according to process requirement.
Special function module
• In addition to the above listed module, the other frequently
used module for special function interface module,
communication, counter etc. these can be selected according to
process requirement.
2.7. Bus system

• Is a path for the transmission of signals. In the programmable

controllers, it is responsible for the signal exchange between
processor and input/output modules.
• The bus comprises of several signal lines i.e. wires/tracks.
• There are three buses in PLC
1. Address bus which enables the selection of a memory
location or a module
2. Data bus which carries the data from modules to processor
and vice versa
3. Control bus which transfers control and timing signals for
the synchronization of the CPU’s activities within the
programmable controller.
PLC Communications
Serial Communications
•
PLC communications facilities normally provides serial transmission of
information.
Common Standards
RS 232
•
Used in short-distance computer communications, with the majority of
computer hardware and peripherals.
•
Has a maximum effective distance of approx. 30 m.
Local Area Network (LAN)
•
Local Area Network provides a physical link between all devices plus
providing overall data exchange management or protocol, ensuring that each
device can “talk” to other machines and understand data received from them.
•
LANs provide the common, high-speed data communications bus which
interconnects any or all devices within the local area.
•
LANs are commonly used in business applications to allow several users to
share costly software packages and peripheral equipment such as printers and
hard disk storage.
2.8. Working of PLC
While the PLC is running, the scanning process includes the
following four phases, which are repeated continuously as
individual cycles of operation:
PHASE 1
Read Inputs
Scan
PHASE 2
Program
Execution
PHASE 3
Diagnostics/
Comm
PHASE 4
Output
Scan
• Phase 1 – Input Status scan
 A PLC scan cycle begins with the CPU reading the status of its
inputs.
• Phase 2– Logic Solve/Program Execution
 The application program is executed using the status of the
inputs
• Phase 3– Diagnostics/ Comm
 Once the program is executed, the CPU performs
diagnostics and communication tasks
• Phase 4 - Output Status Scan
 An output status scan is then performed, whereby the
stored output values are sent to actuators and other field
output devices. The cycle ends by updating the outputs
• As soon as Phase 4 are completed, the entire cycle begins
again with Phase 1 input scan.

• The time it takes to implement a scan cycle is called

SCAN TIME. The scan time composed of the program
scan time, which is the time required for solving the
control program, and the I/O update time, or time
required to read inputs and update outputs. The program
scan time generally depends on the amount of memory
taken by the control program and type of instructions
used in the program. The time to make a single scan can
vary from 1 ms to 100 ms.
Four main steps that shows PLC working principle
1. Bringing input signal status to the internal memory of
CPU
• As mentioned earlier, the field signals are connected to input
module. At the output of input module the field status
converted into the voltage level required by the CPU is
always available.
• At the beginning of each cycle the CPU brings in all the field
input signals from input module and stores into its internal
memory as process image of input signal. This internal
memory of CPU is called as PII, meaning Process Image
Input.
The programmable controller operates cyclically meaning
when complete program has been scanned; it starts again at
the beginning of the program.
2. Processing of signal using program

• Once the field input status is brought in to the internal memory

of CPU i.e. in the PII, the execution of user program begins,
statement by statement. Based on the user program the CPU
performs logical and arithmetic operations on the data from
PII. It also processes times and counts as well as flag states.

I/O
bus CPU
Field
signal Input
s Module

PII PIQ
3. Storing the results of processing in the internal
memory
• The results of the user program scan are then stored in the
internal memory of CPU. This internal memory is called
Process Output Image or PIQ

User program
memory

Internal Timers

CPU
Internal counters
PII PIQ Flags
Internal Relays (Flags)
• Internal relays are imaginary programmed relays that are not
connected to any real output but have coils and contacts. Since
they are not addresses for real outputs, internal relays cannot
have addresses that start with "Q".
• Instead they have addresses of the user selection that starts
with "M" like "M0.0" or "M1.3".
• Of course there is a certain number of coils allowed to be
programmed in SIMATIC S7 software which is 2048 internal
relay. Internal relays have many uses.
• One of their common uses is the case when a certain condition
must be used in the operation of more than one rung
(network).
4. Sending process output to output module
• At the end of the program run i.e. at the end of scanning cycle,
the CPU transfers the signal states in the process image output
to the output module and further to field controls.

CPU
Output Field
Module Control
PII PIQ s
 Selecting a PLC
Several factors are used for evaluating the quality and
performance of programmable controllers when selecting a unit
for a particular application. These are listed below.

 Number of I /O Ports
This specifies the number of I/O devices that can be connected to
the controller. There should be sufficient I/O ports to meet
present requirements with enough spares to provide for moderate
future expansion.

 Output-port Power Ratings

Each output port should be capable of supplying sufficient
voltage and current to drive the output peripheral connected to it.
 Scan Time
• This is the speed at which the controller executes the relay-
ladder logic program. This variable is usually specified as the
scan time per 1000 logic nodes and typically ranges from 1 to
200 ms
 Memory Capacity
• The amount of memory required for a particular application is
related to the length of the program and the complexity of the
control system. Simple applications having just a few relays
do not require significant amount of memory. Program length
tend to expand after the system have been used for a while. It
is advantageous to a acquire a controller that has more
memory than is presently needed
 List of items required when working with PLCs:
1. Programming Terminal - laptop or desktop PC.
2. PLC Software. PLC manufacturers havetheir own specific
software and license key.
3. Communication cable for connection from Laptop to PLC.
4. Backup copy of the ladder program (on diskette, CDROM, hard
disk, flash memory). If none, upload it from the PLC.
5. Documentation- (PLC manual, Software manual, drawings, ladder
program printout, and Seq. of Operations manual.)
A Detailed Design Process
1. Understand the process
2. Hardware/software selection
3. Develop ladder logic
4. Determine scan times and memory requirements
PLC Status Indicators
• Power On
• Run Mode
• Programming Mode
• Fault